1. Technical Field
The present disclosure relates to a measuring method. More particularly, the present disclosure relates to a method for thickness calibration and for measuring the thickness of a first absorbing material in the presence of a second absorbing material.
2. Description of Related Art
With the evolution of electronic technology, electronic devices become more complicate then ever before but with its size getting smaller and lighter. In is order to maintain the stability and correctness of the data transmission, a careful and precise inspection is needed to perform on the soldering and assembly of electronic devices.
In conventional design, an X-ray system is used to make the inspection. The images or pictures formed represent the X-ray shadow cast by an object being inspected when it is illuminated by a beam of X-rays. The X-ray shadow is detected and recorded by an X-ray sensitive material such as film or electronic means. In automated X-ray inspection of printed circuit assemblies, gray-scale images of interconnects or slices thereof are examined to detect improper joints and/or to provide statistical process control data relating to the manufacturing process. In characterizing solder joints, for example, it is preferable to deal with measured joint thickness rather than gray scale pixel values. However, extracting solder thickness from the measure gray scale pixel values is complicated by several factors. For example, in passing through a printed circuit assembly, X-rays will typically encounter other absorbers in addition to the solder, e.g. copper ground planes, tantalum capacitors, etc. Each material has its own characteristic absorption spectrum. The resulting interaction is highly non-linear, and complete characterization of the thickness of the solder and other shading materials in the path is generally not possible from a limited number of gray scale calibration measurements.
Some approximation methods can be used to estimate the thickness of the solder such as U.S. Pat. No. 6,201,850 entitled “ENHANCED THICKNESS CALIBRATION AND SHADING CORRECTION FOR AUTOMATIC X-RAY INSPECTION”. However, the lack of the theoretical basis of the hypothesis of the non-linear relationship between the gray scale pixel values and the X-ray energy may lead to an inaccuracy of the measurement. In addition, using the look-up table constructed after the calibration process to estimate the thickness of the solder is time-consuming.
Thus, what is needed is a quick and precise method for measuring the thickness of material in a precise way. The present disclosure addresses the need.